The present invention relates to an improved self drilling anchoring dowel fixable inside a hard support structure and having an elongated substantially cylindrical body and a coaxial bore therein and provided at its front end with an indented drilling head and at its rear end with a rear terminal section.
Where mention is made herein of a hard support structure, it is to be understood to mean a structure made of, or consisting in, a strong material, such as rock, concrete, masonry work or any other material of hard characteristics, but substantially brittle.
Self drilling anchoring dowels of this type known in the art are constructed so that they are coupled by means of special instruments via holding cones with pressurized air drill hammers and are drilled into the mass of a hard structure by combined hammering and drilling motions. The depth of the hole drilled is determined by the configuration of the special instrument utilized. After a hole of a desired depth has been drilled, the hammer drill is pulled out from the drilled hole, together with the instrument whereto it is coupled.
During the drilling operation, the drilled hole must be cleaned to force out of it any chips and dust produced by the drilling operation. This is performed, according to the prior art, by air blasting. However, blasting causes a great amount of dust to be expelled from the drilled hole and dispersed through the surrounding air. The dust dissipates far throughout the working area and may exceed the maximum allowable concentration in the atmosphere, so that the operator may suffer therefrom and be exposed to health risk when inhaling the dust.
As an alternative, it has proved to be very useful to exhaust the particulate material through a central bore provided in the drilling dowel.
In the prior art, self drilling anchoring dowels have been known, which dowels had along their entire length an axial bore and an unchanging diameter. The bore was often slightly enlarged at the front inlet of the drilling head. The front side of the dowel body was provided at its drilling head with more or less steep teeth sloping towards the center of the dowel body.
These known constructions have not proven to be efficient. Even if provided with a coaxial bore arranged for withdrawal the particulate material, these dowels of the known constructions have been ineffective in regard to exhaustion from the drilled hole of the dust originated by the drilling operation. This shortcoming results from the fact that, during the drilling, there are broken off from the hard material pieces of lens-like form known as "stone lenses". The lenses are drawn by the applied suction into the bore of the dowel body. The diameters of these lenses are usually nearly as great as, or only slightly smaller than, the diameter of the bore existing in the drilling head of the dowel. It has been already mentioned that in the known self drilling anchoring dowels the drilling head has the front inlet to the bore slightly enlarged. Thus, the pieces called "stone lenses" tend to become wedged in the bore behind the enlarged inlet, since here the bore has a diameter smaller than that existing in the drilling head. Moreover, all the otherwise configurated chips broken off by drilling accumulate over and around these lenses until the dowel bore becomes totally clogged. When this occurs, the exhaustion of the drilling waste material and of dust is interrupted and further drilling is hampered.